Propylene-type polymers are excellent in heat resistance, chemical resistance, and rigidity, and have further excellent properties which polystyrene and polyethylene do not have. Polystyrene and polyethylene are widely used as raw materials of foamed substances in various fields, in which their properties are respectively exhibited. However, propylene-type polymers, although having the above-mentioned excellent properties, have not been fully utilized in the field of foaming.